1. Field of the Invention
The present invention relates to a control method for an electronically controlled thermostat capable of optionally varying the valve-opening ratio by installing a heater element in a temperature sensor, for example, in a cooling-water temperature control system for an internal combustion engine (hereinafter referred to as ‘engine’) that is employed in an automobile or the like.
2. Description of the Related Art
A water-cooling-type cooling device that employs a radiator is generally used in order to cool an automobile engine. Further, conventionally, with the objective of improving the fuel consumption of the automobile, this type of cooling device employs a control valve, such as a thermostat, for example, which adjusts the amount of cooling water circulated to the radiator to permit control of the temperature of the cooling water introduced to the engine. Known examples of such thermostats include those that employ a thermally expanding body (WAX or the like) as a temperature sensor or those that are electrically controlled, and so forth.
A thermostat of this kind is constituted such that the valve portion thereof is interposed in part of a cooling water passage. When the cooling water temperature is low, the valve portion is closed so that cooling water is circulated via a bypass passage without passing through the radiator, and, when the cooling water temperature is high, the valve portion is opened so that the cooling water is circulated via the radiator, whereby the temperature of the cooling water is controlled to the required state.
Further, it is generally known that automobile fuel consumption is improved by raising the cooling water temperature of the automobile engine.
In view of this situation, most recently, electronic-control type valves, that is, electronically controlled thermostats, have been widely adopted in order to provide the optimum water temperature to improve automobile fuel consumption.
Such an electronically controlled thermostat controls the cooling water temperature by optionally controlling the opening ratio of the valve portion and controlling a cooling fan that is attached to the radiator, whereby appropriate control of the cooling water temperature is possible. This is because control device (engine control module) that variably controls the above-mentioned electronically controlled thermostat is capable of performing control also through the addition of detected information such as a variety of parameters of the engine control unit such as the cooling water temperature, the outside air temperature, the automobile speed, the engine rotation speed, and throttle opening ratio, for example.
A multiplicity of different types of thermostat has been proposed conventionally as means for improving fuel consumption or as means for suppressing the generation of overshooting or undershooting by controlling the cooling water temperature at the required state, including an electronically controlled thermostat that is constituted to be capable of rendering the supply of engine-startup cooling water instantaneous and improving engine fuel consumption by mounting a heater element in the temperature sensor of the thermostat, combining power distribution control for this heater element, and using feedback control such as PID control for this power distribution control, for example.
An electronically controlled thermostat that is constructed to calculate the engine load and to control the thermostat valve-opening ratio (valve opening amount) and control the radiator cooling fan in accordance with the cooling water temperature (actual water temperature) by means of various parameters such as the cooling water temperature and automobile speed, for example, has been conventionally proposed (such as Japanese Patent Application Laid-Open No. H11-294164, for example).
Furthermore, an electronically controlled thermostat that is constituted to perform cooling-fan control in the required state by determining the thermostat's respective outputs to the heater and fan by comparing a threshold value with the actual water temperature, has been conventionally proposed (such as Japanese Patent Application Laid-Open No. H11-062584, for example).
Further, in the case of an electronically controlled thermostat in which a heater element is installed in a temperature sensor as mentioned earlier, after the power distribution control for the heater element has distributed power to the heater element installed in the temperature sensor, the response time until the valve opens and feedback to the water temperature occurs poses a problem. In other words, the response time must be made as short as possible in order to control the cooling water in the electronically controlled thermostat to a constant water temperature (hereinafter ‘constant water temperature control’) That is, constant water temperature control, whereby the response time can be shortened, can be implemented by applying a larger power-distribution amount variation to the heater element at an earlier time.
Here, where feedback control such as general PID control is concerned, although early prediction of a water temperature variation is implemented by means of the differential control itself, it is necessary to counteract the response delay of a mechanical part in the electronically controlled thermostat unit and predict a water temperature variation that shortens the time of the response delay. Here, the delay of a mechanical part in the electronically controlled thermostat unit is the time interval until power is distributed to the actuator and the valve operates. Further, a larger power-distribution amount variation cannot be applied to the heater element because the power distribution amount is an amount proportional to the water temperature gradient.
Therefore, for the reasons listed above, it is difficult to implement constant water temperature control of the electronically controlled thermostat by means of general PID control.
Furthermore, it is generally known that the thermostat open-valve temperature can also be set at a high temperature in order to implement the fuel consumption improvements of a higher water temperature. However, in the case of a conventional WAX-type thermostat control method, the difference in temperature between the thermostat open-valve temperature and fan-operating temperature is large (10° C. or more, for example). This is because the fan operating temperature has the characteristic of being set at the temperature at which the thermostat is fully open and the fan operating temperature is fixed at a temperature at which the engine does not break down, and it is therefore difficult to set the thermostat fully-open temperature boundlessly at a high value.